Steam sterilization of autoclavable pelleted rodent diets increased pellet hardness which may result in prolonged intervals between litters, reduced weight gain, and reduced litter size. Feed pellets from different diets and pellets from the same diet formula manufactured by different rodent diet vendors may vary in size, shape, texture and hardness. Normal pellet hardness measurements of commercially available rodent diets have not been reported. Test diets were purchased from rodent diet vendors A, B, C and D. Diets were autoclaved at 250 degrees F at 15-18 lbs. pressure for 20 minutes. Results from 90 pellets (3 replicates of 30) from each mill date for each diet were compared for pellet hardness. Mean pellet hardness values for nonautoclavable diets are given in parentheses. Mean pellet hardness values for the pre and post autoclaved diets are given in brackets: Vendor A NIH-07 (78.01 plus\minus 11.98), NIH-31 [73.49 plus/minus 9.86], [94.79 plus\minus 12.46]; Vendor B NIH-07 (42.82 plus\minus 7.67), NIH-31 [41.21 plus\minus 7.18], [55.66 plus\minus 11.37]; Vendor C NIH-07 (82.91 plus\minus 22.85), NIH-31 [84.17 plus\minus 19.16], [93.16 plus\minus 26.13], #7012- LM-485 [41.90 plus\minus 6.48], [43.23 plus\ minus 8.30], #8626 (43.97 plus\minus 5.34); Vendor D NIH-31 [42.09 plus\minus 8.07], [47.93 plus\minus 10.24], #5001 (56.99 plus\minus 7.91), #5002 (82.58 plus\minus 17.52), #5015 (48.98q6.29), #5058 (45.22q6.06), #5010 [42.77q6.81], [46.69 plus\minus 9.11], and #5021 [38.78 plus\minus 6.86], [45.90 plus\minus 9.43]. Moisture content was similar for both nonautoclaved and autoclaved diets. It was concluded that: rodent diets differ in pellet hardness, pellets from the same diet processed by different rodent diet vendors differ in pellet hardness, steam sterilization of diets increases pellet hardness, and in order to compare pellet hardness measurements from different diets, a standardized procedure must be used and the diet's vendor specified.